Avalanche Forecast
Regions: Cascades - North East.
Heightened avalanche danger exists on wind-loaded slopes near and above treeline. Watch for fresh drifts, wind stiffened snow, and slick old snow surfaces. Beware of places where persistent weak layers still linger, especially high north facing terrain and areas with a shallow snowpack.
Discussion
Snow and Avalanche Discussion
Since Friday, the Washington Pass area received 4-6in of new snow with steady southwest winds. The new snow is not bonding well to variable old snow surfaces. Small human-triggered avalanches failing at the old/new snow interface were reported the past two days. On Saturday, observers in the Varden Creek area found fresh wind slabs 6-12in thick on leeward slopes near and above treeline. Scoured snow surfaces and newly formed drifts of snow were easy to find.Â
Snowpack Discussion
January, 31, 2019
As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week. This break in the weather allowed for avalanche danger to steadily decline in all regions. Stormy weather starting February 1st once again elevated the avalanche danger and brought a round of storm related avalanches.
We’ve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it has landed on slick surfaces. It's time to pay attention to the interfaces formed by our recent rounds of precipitation.
A skier triggered storm slab in the recent snow in a steep southeast start zone at 5700ft. Lichtenberg Mtn. Feb 2. Photo: Josh Hirshberg
North-South:
While a high elevation rain event, around January 23rd, formed surface crust in many regions, it’s the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.
Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara
North: On shady slopes, things haven’t exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes haven’t been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low to mid elevations, rain may have melted any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at the interface buried around February 1st.
Roller balls and loose wet avalanches on a sunny, southeast slope following the Feb 1st storm Lichtenberg Mtn. Photo: Josh Hirshberg
South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them. Following the Feb 1st storm, the sun drove a round of small wet loose avalanches and storm slabs on upper elevations slopes that received enough accumulation.
East-West:While we're tracking persistent weak layers (PWL’s), they haven't been the source of avalanches for over a week. It’s not uncommon for our east-side forecast zones to experience lingering PWL's. This season, we’ve also seen several different PWL’s in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.
West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout  crust, and strong frozen melt-forms make up the majority of the snowpack at this time.
Southeasterly winds textured surfaces on the east side of Washington Pass on Feb 1-2nd. Photo Matt Primomo
East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers we’ve got our eyes on.
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January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .
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December 26th surface hoar. This layer can be found from 16†to 40†down and is still producing clean, planar shears with tests.
You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been “dormant†or unreactive during the week of high pressure. The latest storm has not been enough to re-activate theses weak layers. We’ll keep tracking them to watch their progression..
The lower eastern slopes and the Columbia River. Snow exists at low elevations, but snowpack depths are shallow. Photo: Matt Primomo
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Avalanche Problems
Wind Slabs
Recent snowfall and steady southwest winds have formed shallow wind slabs at mid and upper elevations. While any avalanches will most likely be small, they may be easy to trigger. Watch for blowing snow and fresh drifts on leeward sides of ridges. Be careful of convex slopes 35 degrees and steeper near and above treeline. Use small test slopes and quick tests, like a shovel tilt test, to check how the new snow is bonding to old surfaces.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
In the Washington Pass area, the snowpack is generally strong and well settled. However, a few persistent weak layers still exist in the top three feet of the snowpack. You are most likely to find these preserved on shaded and sheltered open slopes. Near and east of Mazama, the snowpack may be shallow. Where you find less than 4 feet of total snow, suspect a weaker snowpack structure and lingering weak layers near the ground. Watch for shooting cracks in the snow, and listen for collapses. Use the terrain to your advantage, avoiding areas with thick slabs adjacent to shallow, rocky slopes. Use extra caution around steep, extreme terrain and unsupported slopes on shaded aspects.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, South East, South West, West, North West.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1